Level order traversal with direction change after every two levels | Recursive Approach

Given a binary tree, print the level order traversal in such a way that first two levels are printed from left to right, next two levels are printed from right to left, then next two from left to right and so on. So, the problem is to reverse the direction of level order traversal of the binary tree after every two levels.

Examples:

Input:
1
/   \
2       3
/  \     /  \
4    5    6    7
/ \  / \  / \  / \
8  9 3   1 4  2 7  2
/     / \    \
16    17  18   19
Output:
1
2 3
7 6 5 4
2 7 2 4 1 3 9 8
16 17 18 19
In the above example, the first two levels
are printed from left to right, next two
levels are printed from right to left,
and then the last level is printed from
left to right.

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: In the previous post, level order traversal using queue and stack has been done to print the elements. A recursive method has been used over here to print the elements in every level. Traverse every level in the tree, for every level, check the direction. Use a flag to know the direction of traversal in the tree. If the flag is set to true, print the nodes from right to left in the particular level. If the flag is set to false, print the nodes in that level from left to right. Initially, the flag is set to False, after every 2 levels, flag changes its value to true and vice versa.

Below is the implementation of the above approach.

C++

 // C++ program level order traversal // with direction change // after every two levels #include using namespace std;    struct node {     int data;     node *left, *right; } * temp;    // inserts new node node* newNode(int data) {     temp = new node;     temp->data = data;     temp->left = temp->right = NULL;        return temp; }    // function to  print current level void printCurrLevel(node* root, int level, bool flag) {     if (!root)         return;        if (level == 1) {         cout << root->data << " ";         return;     }        else {         // If the flag is true, we have to print         // level from RIGHT to LEFT.         if (flag) {             printCurrLevel(root->right, level - 1, flag);             printCurrLevel(root->left, level - 1, flag);         }            // If the flag is false, we have to print         // level from LEFT to RIGHT.         else {             printCurrLevel(root->left, level - 1, flag);             printCurrLevel(root->right, level - 1, flag);         }     } }    // This function returns the height of tree. int height(node* root) {     if (!root)         return 0;        // left subtree     int lh = height(root->left);        // right subtree     int rh = height(root->right);        return 1 + max(lh, rh); }    // Fucntion to traverse level-wise and // print nodes void modifiedLevelOrder(node* root) {     int h = height(root);        // Variable to choose direction.     bool flag = false;     for (int i = 1; i <= h; i++) {         printCurrLevel(root, i, flag);         cout << endl;            // change direction after every two levels.         if (i % 2 == 0)             flag = !flag;     } }    // Driver Code int main() {        // create tree that is given     // in the example     node* root = newNode(1);     root->left = newNode(2);     root->right = newNode(3);     root->left->left = newNode(4);     root->left->right = newNode(5);     root->right->left = newNode(6);     root->right->right = newNode(7);     root->left->left->left = newNode(8);     root->left->left->right = newNode(9);     root->left->right->left = newNode(3);     root->left->right->right = newNode(1);     root->right->left->left = newNode(4);     root->right->left->right = newNode(2);     root->right->right->left = newNode(7);     root->right->right->right = newNode(2);     root->left->right->left->left = newNode(16);     root->left->right->left->right = newNode(17);     root->right->left->right->left = newNode(18);     root->right->right->left->right = newNode(19);        modifiedLevelOrder(root);     return 0; }

Java

 // Java implementation of above idea  import java.util.*;    class GFG  {        static class node  {      int data;      node left, right;  }  static node temp;     // inserts new node  static node newNode(int data)  {      temp = new node();      temp.data = data;      temp.left = temp.right = null;         return temp;  }     // function to print current level  static void printCurrLevel(node root, int level, boolean flag)  {      if (root == null)          return;         if (level == 1)      {              System.out.print(root.data + " ");              return;      }         else     {          // If the flag is true, we have to print          // level from RIGHT to LEFT.          if (flag)          {              printCurrLevel(root.right, level - 1, flag);              printCurrLevel(root.left, level - 1, flag);          }             // If the flag is false, we have to print          // level from LEFT to RIGHT.          else          {              printCurrLevel(root.left, level - 1, flag);              printCurrLevel(root.right, level - 1, flag);          }      }  }     // This function returns the height of tree.  static int height(node root)  {      if (root == null)          return 0;         // left subtree      int lh = height(root.left);         // right subtree      int rh = height(root.right);         return 1 + Math.max(lh, rh);  }     // Fucntion to traverse level-wise and  // print nodes  static void modifiedLevelOrder(node root)  {      int h = height(root);         // Variable to choose direction.      boolean flag = false;      for (int i = 1; i <= h; i++)      {          printCurrLevel(root, i, flag);          System.out.println("");            // change direction after every two levels.          if (i % 2 == 0)              flag = !flag;      }  }     // Driver Code  public static void main(String[] args) {     // create tree that is given      // in the example      node root = newNode(1);      root.left = newNode(2);      root.right = newNode(3);      root.left.left = newNode(4);      root.left.right = newNode(5);      root.right.left = newNode(6);      root.right.right = newNode(7);      root.left.left.left = newNode(8);      root.left.left.right = newNode(9);      root.left.right.left = newNode(3);      root.left.right.right = newNode(1);      root.right.left.left = newNode(4);      root.right.left.right = newNode(2);      root.right.right.left = newNode(7);      root.right.right.right = newNode(2);      root.left.right.left.left = newNode(16);      root.left.right.left.right = newNode(17);      root.right.left.right.left = newNode(18);      root.right.right.left.right = newNode(19);         modifiedLevelOrder(root);      } }    // This code is contributed by Princi Singh

Python3

 # Python3 program level order traversal with # direction change after every two levels  class Node:             def __init__(self, data):         self.data = data         self.left = None         self.right = None        # function to print current level  def printCurrLevel(root, level, flag):         if root == None:         return        if level == 1:          print(root.data, end = " ")          return        else:                    # If the flag is true, we have to          # print level from RIGHT to LEFT.          if flag:              printCurrLevel(root.right,                             level - 1, flag)              printCurrLevel(root.left,                             level - 1, flag)             # If the flag is false, we have to          # print level from LEFT to RIGHT.          else:             printCurrLevel(root.left,                             level - 1, flag)              printCurrLevel(root.right,                             level - 1, flag)             # This function returns the  # height of tree.  def height(root):         if root == None:         return 0        # left subtree      lh = height(root.left)         # right subtree      rh = height(root.right)         return 1 + max(lh, rh)     # Function to traverse level-wise  # and print nodes  def modifiedLevelOrder(root):         h = height(root)         # Variable to choose direction.      flag = False     for i in range(1, h + 1):          printCurrLevel(root, i, flag)          print()             # change direction after every          # two levels.          if i % 2 == 0:              flag = not flag     # Driver Code  if __name__ == "__main__":         # create tree that is given      # in the example      root = Node(1)      root.left = Node(2)      root.right = Node(3)      root.left.left = Node(4)      root.left.right = Node(5)      root.right.left = Node(6)      root.right.right = Node(7)      root.left.left.left = Node(8)      root.left.left.right = Node(9)      root.left.right.left = Node(3)      root.left.right.right = Node(1)      root.right.left.left = Node(4)      root.right.left.right = Node(2)      root.right.right.left = Node(7)      root.right.right.right = Node(2)      root.left.right.left.left = Node(16)      root.left.right.left.right = Node(17)      root.right.left.right.left = Node(18)      root.right.right.left.right = Node(19)         modifiedLevelOrder(root)     # This code is contributed by Rituraj Jain

C#

 // C# implementation of above idea  using System;    class GFG  {         public class node  {      public int data;      public node left, right;  }  static node temp;     // inserts new node  static node newNode(int data)  {      temp = new node();      temp.data = data;      temp.left = temp.right = null;         return temp;  }     // function to print current level  static void printCurrLevel(node root, int level, Boolean flag)  {      if (root == null)          return;         if (level == 1)      {          Console.Write(root.data + " ");          return;      }         else     {          // If the flag is true, we have to print          // level from RIGHT to LEFT.          if (flag)          {              printCurrLevel(root.right, level - 1, flag);              printCurrLevel(root.left, level - 1, flag);          }             // If the flag is false, we have to print          // level from LEFT to RIGHT.          else         {              printCurrLevel(root.left, level - 1, flag);              printCurrLevel(root.right, level - 1, flag);          }      }  }     // This function returns the height of tree.  static int height(node root)  {      if (root == null)          return 0;         // left subtree      int lh = height(root.left);         // right subtree      int rh = height(root.right);         return 1 + Math.Max(lh, rh);  }     // Fucntion to traverse level-wise and  // print nodes  static void modifiedLevelOrder(node root)  {      int h = height(root);         // Variable to choose direction.      Boolean flag = false;      for (int i = 1; i <= h; i++)      {          printCurrLevel(root, i, flag);          Console.WriteLine("");             // change direction after every two levels.          if (i % 2 == 0)              flag = !flag;      }  }     // Driver Code  public static void Main(String[] args)  {      // create tree that is given      // in the example      node root = newNode(1);      root.left = newNode(2);      root.right = newNode(3);      root.left.left = newNode(4);      root.left.right = newNode(5);      root.right.left = newNode(6);      root.right.right = newNode(7);      root.left.left.left = newNode(8);      root.left.left.right = newNode(9);      root.left.right.left = newNode(3);      root.left.right.right = newNode(1);      root.right.left.left = newNode(4);      root.right.left.right = newNode(2);      root.right.right.left = newNode(7);      root.right.right.right = newNode(2);      root.left.right.left.left = newNode(16);      root.left.right.left.right = newNode(17);      root.right.left.right.left = newNode(18);      root.right.right.left.right = newNode(19);         modifiedLevelOrder(root);  }  }     /* This code is contributed by PrinciRaj1992 */

Output:

1
2 3
7 6 5 4
2 7 2 4 1 3 9 8
16 17 18 19

My Personal Notes arrow_drop_up A Coding Enthusiast Rails Developer

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.